1. Field of the Invention
The present invention relates to a technique for setting up parameters which are used in a manufacturing process of a semiconductor device and a fine element of an integrated circuit.
2. Description of the Background Art
FIG. 1 is a conceptual diagram of a conventional segregation model which is now in use in a manufacturing process of a semiconductor device. In FIG. 1, a position z is measured along the horizontal axis, and an impurity concentration is measured along the vertical axis as a function of the position z. SiO.sub.2 (oxide film) is in the region 1 where the position z has a negative value and Si (silicon) is in the region 2 where the position z has a positive value. Two different impurity concentrations C.sub.1 and C.sub.2 are set at an interface, where the position z is zero. The values C.sub.1 and C.sub.2 are impurity concentrations of the oxide film and silicon, respectively, at the interface.
In FIG. 1, the dotted line represents a dopant flux Fs (density of flow 1/(cm.sup.2 .multidot.scc)) across the interface.
The conventional segregation model assumes that the interface has no range. Hence, as shown in FIG. 1, a concentration C of such doping species as B (boron), P (phosphorus) and As (arsenic) is discontinuous across the interface, and so are the densities of SiO.sub.2 (oxide film) and Si (silicon).
Using this segregation model, the dopant flux Fs across the interface is Fs=h.multidot.(C.sub.1 -C.sub.2 /m) where h is a mass-transfer coefficient (cm/sec), and m is a segregation coefficient (dimensionless).
The equation above states that the interfacial dopant flux Fs is created in such a manner that a ratio of the impurity concentrations C.sub.2 /C.sub.1 is equal to the segregation coefficient m.
The conventional segregation model, which gives different impurity concentrations at the same position (z=0), or the interface, has grave disagreement with a physical phenomenon: that is, a distribution of impurity concentration gradually ranges from ten angstrom to several tens angstrom across an SiO.sub.2 /Si interface.
Hence, the conventional segregation model is not practical enough for parameter setup to accurately set an impurity concentration near an interface, e.g., to adjust a threshold voltage V.sub.th of a MOS transistor within a predetermined range.
In order to predict a diffusion profile with such accuracy using the conventional segregation model, parameters for different manufacturing process steps must be set by computing an impurity concentration for each manufacturing process step in light of the mass-transfer coefficient h and the segregation coefficient m.